The present invention relates to a slurry for chemical mechanical polishing used in fabrication of a semiconductor device, and more particularly to a slurry for chemical mechanical polishing well suited to use in formation of a buried metal interconnection wherein a tantalum-based metal is utilized as a material for a barrier metal film.
In formation of a semiconductor integrated circuit such as an ULSI (Ultra Large Scale Integrated circuit) for which progress to attain further miniaturization and more densely spaced arrangement has been, in recent years, gathering more speed, copper has been attracting strong attention as a particularly useful material for the electric connection due to its excellent electromagnetic resistance and considerably low electrical resistance.
A copper interconnection is currently formed, due to problems such as a difficulty to make patterning through dry etching, in the following way. That is, after a sunken section such as a trench or a connection hole is formed in an insulating film and a barrier metal film is formed thereon, a copper film is grown by the plating method so as to fill up the sunken section, and then by conducting the chemical mechanical polishing (referred to as xe2x80x9cCMPxe2x80x9d hereinafter) until the surface of the insulating film other than the sunken section is completely exposed, the surface is planarized, and thereby formation of electric connection sections such as a buried copper interconnection which is substantially made of copper filling the sunken section, a via plug, a contact plug and the like are accomplished.
Now, with a reference to FIG. 1, a method of forming a buried copper interconnection is described below.
Firstly, on a silicon substrate where a semiconductor device is formed (not shown in the drawing), there is formed a lower interconnection layer 1 made of an insulating film having a lower interconnection (not shown in the drawing). Thereon, a silicon nitride film 2 and a silicon oxide film 3 are formed consecutively in this order, as shown in FIG. 1(a), and then in the silicon oxide film 3 a sunken section in the form of an interconnection pattern is formed to reach the silicon nitride film 2.
Next, as shown in FIG. 1(b), a barrier metal film 4 is formed by the sputtering method. On the film, a copper film 5 is applied to the entire surface by the plating method so as to fill up the sunken section.
After that, as shown in FIG. 1(c), the copper film 5 is polished by means of CMP to planarize the substrate surface. Polishing by the CMP continues until the metal over the silicon oxide film 3 is completely removed, as shown in FIG. 1(d).
In such formation of a buried copper interconnection as described above, a barrier metal film is formed as a base film, for the purpose of preventing diffusion of copper into the insulating film and the like. However, when a tantalum-based metal such as Ta or TaN is employed as a barrier metal film, there may arise a problem that with a conventional polishing slurry the polishing rate for the barrier metal film made of Ta or TaN becomes smaller than that for the copper film, owing to the substantially high chemical stability of Ta and TaN. Specifically, when formation of a buried copper interconnection or such is carried out using the CMP with a conventional polishing slurry, a considerable difference between rates for the copper film and the barrier metal film are produced, which may bring about dishing and erosion.
Dishing is a phenomenon that copper in the sunken section is excessively polished so that the center of the copper film in the sunken section falls back with respect to the plane of the insulating film laid on the substrate, as shown in FIG. 2. A conventional polishing slurry requires an ample polishing time to remove the barrier metal film 4 lying on the insulating film (silicon oxide film 3) thoroughly because of the considerably low polishing rate for the barrier metal film. The polishing rate for the copper film 5 is, however, higher than that for the barrier metal film 4, so that the copper film 5 becomes excessively polished to create dishing.
Erosion is, on the other hand, a phenomenon that polishing in a densely-spaced interconnection region proceeds excessively, compared with that in a region with a low interconnection density such as an isolated interconnection region, so that the surface of the densely-spaced interconnection region falls back with respect to the surface of other regions, as shown in FIG. 1(d). When, in a wafer, the densely-spaced interconnection region where many buried sections formed of the copper film 5 are present is considerably separated from the isolated interconnection region where a few buried section formed of the copper film 5 are present by, for example, a region without any interconnections, and the copper film 5 is polished faster than the barrier metal film 4 or a silicon oxide film 3 (an insulating film), then, in the densely-spaced interconnection region, a polishing pad pressure to the barrier metal film 4 or the silicon oxide film 3 becomes higher than that in the isolated interconnection region. As a result, in the CMP step after exposing the barrier metal film 4 (the step of FIG. 1(c) and thereafter), there is produced a difference in polishing rate by the CMP between the densely-spaced interconnection region and the isolated interconnection region so that the insulating film in the densely-spaced interconnection region is excessively polished to create erosion.
Dishing created in the step of forming an electric connection section in a semiconductor device as described above may cause an increase in interconnection resistance and connection resistance as well as an increase in electron migration liability, which may lower the reliability of the device. The creation of erosion may also adversely affect the evenness of the substrate surface, the effect of which becomes more prominent in a multi-layered structure so that problems such as an increase and a variation in interconnection resistance may arise.
So far, various investigations have been made to overcome these problems.
For example, in JP-A 238709/1999, it is disclosed that dishing in the CMP step may be prevented by using a polishing slurry which contains benzotriazole or its derivative and thereby forming a protective film on a copper surface.
Further, in JP-A 44047/1998, it is disclosed that, when the CMP is conducted using a polishing slurry containing a particular organic acid such as acetic acid, a difference in polishing rate between an aluminium layer for interconnection and a silicon oxide film can be increased, and besides a removal rate for a titanium film as a barrier metal film can be heightened.
Further, in JP-A 46140/1998, it is disclosed that the use of a composition for the CMP which contains a particular carboxylic acid such as malic acid, an oxidizing agent and water, whose pH is adjusted to 5-9 by an alkali, can improve the polishing rate and, at the same time, prevent dishing associated with the corrosion mark.
However, when, to form a buried copper interconnection having a tantalum-based barrier metal film, the CMP is conducted, there are occasions even the use of one of the polishing slurries disclosed in the above publications cannot prevent dishing and erosion from occurring, satisfactorily.
Meanwhile, with the object of improving efficiency of the polishing slurry, addition of the amino acid has been the subject of investigation.
For example, in JP-A 83780/1996, it is disclosed that the use of an amino acid as an etchant for a material whose main component is a metal such as copper can prevent dishing. Therein, glycine that is a neutral amino acid is given as an example of the amino acid.
Further, in JP-A 21546/1999, it is described that an amino acid can be added as a complex-forming agent to disturb a passive layer that may be formed by a film-forming agent such as benzotriazole, and besides, in Examples thereof, glycine that is a neutral amino acid is mentioned.
Further, in JP-A 216345/1995, as examples of an organic acid to add into a composition for polishing, there are given amino acids, and more specifically glycine that is a neutral amino acid and glutamic acid that is an acidic amino acid.
In addition, in JP-A 109799/2000, it is disclosed that a polyamino acid can be used as a biodegradable additive for polishing a silicon oxide film that is formed on a stopper film made of silicon nitride or the like, and that, as a monomer for the polyamino acid, basic amino acids such as arginine, histidine and lysine can be employed. However, the addition of an amino acid itself, which is a monomer thereof, is not mentioned in that publication.
As described above, in those publications, the use of amino acids and peptides are described. Nevertheless, the use of a basic amino acid is not particularly mentioned. Further, even if any polishing slurry disclosed in the publications is utilized, dishing and erosion in a buried copper interconnection cannot be necessarily suppressed satisfactorily.
Further, in JP-A 108887/1992, the addition of amino acids to a composition for a polishing agent is described, and therein not only neutral amino acid and acidic amino acid but also basic amino acid are mentioned as amino acids. However, this description is not illustration of particular amino acids but rather a mere general classification of amino acids. Specifically, as examples of amino acids, this publication gives glycine, alanine and amino capronic acid, all of which are neutral amino acids, as well as asparagic acid and glutamic acid that are acidic amino acids, but no basic amino acids are included. Further, in Examples, only glycine is employed. In addition, this publication neither mentions advantages that can be obtained by selecting amino acids, and especially basic ones, nor suggests that the use of basic amino acid can reduce the polishing rate of the tantalum-based barrier metal film and, therefore, suppress the occurrence of dishing and erosion in the buried copper metal interconnection section satisfactorily.
As described above, although a number of proposals for adding an amino acid have been made, the amino acid therein are, in general, neutral or acidic ones and no polishing slurry with a basic amino acid is reported. Because of this, the polishing rate for a tantalum-based barrier metal film is, in some cases, reduced and, consequently, the occurrence of neither dishing nor erosion in the buried copper interconnection section is satisfactorily suppressed.
Accordingly, an object of the present invention is to provide a slurry for chemical mechanical polishing, which can suppress the occurrence of dishing and erosion in the CMP and form a buried interconnection with a little variation in interconnection resistance when forming a buried copper interconnection in which a tantalum-based metal is utilized as a barrier metal film.
The present invention relates to a slurry for chemical mechanical polishing to polish a copper-based metal film formed on a tantalum-based metal film, which comprises a polishing grain, an oxidizing agent and a basic amino acid compound.
In the present invention, a copper-based metal refers to copper or an alloy the main component of which is copper, and a tantalum-based metal, tantalum (Ta) or tantalum nitride (TaN).
The polishing slurry of the present invention can reduce the polishing rate for a tantalum-based metal film and, thus, increase the difference of the polishing rates between the tantalum-based metal film and a copper-based metal film so that the function of the tantalum-based metal film as a stopper film (polishing stopper) in polishing a copper-based metal film is enhanced. As a result, in formation of a buried interconnection of a copper-based metal with a barrier metal film of a tantalum-based metal, dishing and erosion which may result from CMP can be prevented from occurring and a buried interconnection of a copper-based metal wherein a variation in interconnection resistance is well suppressed can be formed.